You have weird nightmares, Tia!
Luna smiled as she dispelled a nightmare from a filly, and created a large land full of candy, rainbows, and sunshine for the filly to trot and play around in, at least until they woke up for school tomorrow morning. As she left the dream, confident that the filly wouldn't be troubled by nightmares for this night, she couldn't help but feel a twinge of regret as she left the filly's dreams; she had always enjoyed watching the dreams of other ponies and their actions inside the dreamscape, but she was unable to linger for a long time in any one pony's dream, as there was the possiblity that there was another pony in a particularly dreadful nightmare, somewhere else in Equestria for her to dispel.
It was also of much regret that she didn't seem to be as powerful as she was before in the dreamscape; being sealed away from one of her domains for such a long time had weakened her power over it and caused her to go out of practice. This was an especially difficult thing to come to terms with, not only because she could not display her awesome powers to help ponies, but it also made dispelling nightmares that much harder. Only practicing her once formidable power would help her overcome her current weakness, but there was not much to practice on; years of continued prosperity and peace had caused the ponies of Equestria to become soft and content, with little in the way of nightmares. In fact, fillies and colts were her only reliable source of training, and she was quickly getting bored with creating a candy land for fillies and colts in every nightmare she visited. Her dream form walked down the streets of Canterlot, on the look out for any nightmares that were distressing ponies; at least, any that weren't from little fillies and colts.
A slight twinge towards her right made her turn around and look up; she realized that it was causing her to look towards her sister's balcony. It seemed as if her sister required her assistance; Faust knew that both of her fillies still had nightmares about Discord, among other things. Luna flew up to Celestia's balcony, and after composing herself, phased through the door. One of the nice things about this job was the fact that her dream form was not (entirely) bound by most rules, so trivial things like walking through doors could happen; why she couldn't sink down through the floor, she didn't know, but oh well. One of the disadvantages of this job, however, was that she often was not able to get to dream herself, being busy entering the dreams and nightmares of others; then again, she mused, she could also have gotten nightmares when she could have gotten dreams, so it wasn't all that bad.
She took a moment to gush at how cute her sister looked like when she was sleeping; she was clutching a life sized Twilight plushie in her hooves, and her mane was splayed out, in its natural color hue. She wondered how many Twilight plushies her sister had before sitting down next to her, and noticed a few things that were off with Celestia and a supposed good night's rest; her teeth were bared slightly, and were being clenched none too softly. If the Twilight plushie was the real pony, the poor mare would have been gasping for air, being hugged with the incredible strength that alicorns possessed. Her breathing was erratic, and it was clear from her pillow and sheets that her sister was sweating profusely. Celestia was having a truly terrible dream, and as the nightmare vanquisher and her sister, it was up to her to fix that.
However, she hesitated, and drew back from her sister slightly. Was she strong enough to conquer whatever nightmares that were in her sister's dreams? Her power may very well be inadequate for a nightmare of this scale; her sister did not usually have nightmares, and whenever she did, they usually promised to be overwhelming ones. She raised a hoof, and gently tried to brush her sister's mane; unfortunately, since she was in her dream form, her hoof did nothing do her sister's mane, and instead went through it and a small part of the bed. The simple act (Or lack of) caused her sister's slightly labored breathing to calm down slightly, and Luna took heart in that simple, unconscious gesture on her sister's part; that simple act gave her the confidence to push through, and dive into her sister's nightmare, with the intent on trying her best to make it a normal dream again.
The very first thing that came across Luna's mind when she looked around inside her sister's nightmare was that it was a completely white void with her sister slumped down on the ground, roughly a few feet away; quite the unusual setting for a nightmare. Before she moved towards her, she called upon her dream magic to help blend her into the background; that wasn't hard to do, since everything around her sister's dreamscape was white. Satisfied with her disguise so that she could view her sister's nightmare without disturbing it, Luna trotted over to her, only to have her sister suddenly stiffen, and turn around wildly; her breath caught in her throat as Luna thought that her camouflage was somehow blown, but her sister was staring past Luna. Following her gaze, she made a stifled gasp as she saw the image of Nightmare Moon; her former, darker self grinned, and walked up towards Celestia, who shrank away from her even as Nightmare Moon began to coo in a soft voice.
"It has been a long day coming, sister, for this very moment."
"I'm sorry! I'm sorry, I really am, Lu-"
"LUNA DOES NOT EXIST ANYMORE, YOU PATHETIC FOAL; THERE IS ME, AND ONLY ME NOW, NIGHTMARE MOON!"
Instead of the strong, willful sister that Luna knew and loved, Celestia pulled away, as if slapped, and glanced fearfully at Nightmare Moon; her eyes were tearing up, and even from this distance, Luna could see the sadness being radiated from Celestia; the very sight of her sister suffering from this nightmare was eating away at her, but she didn't know if this was just a simple nightmare, or something far, far beyond her current power.
"P-Please...Don't do this..." Nightmare Moon growled, and stepped up even closer to Celestia; Luna, on instinct, began to crouch low, ready to tackle Nightmare Moon, when she realized that interfering with the dream now would only make things worse; directly influencing dream (Or nightmare) events with the pony actually seeing you often made things much worse. Grudgingly, she stood back up, watching Nightmare Moon walk up close enough to strike her again; Luna leaned in forward for a better view, Celestia turned her head away, and Nightmare Moon only grinned as she raised a hoof...
...Only to boop Celestia on the nose.
What.
"Tag, you're it!" Celestia whined as Nightmare Moon bounced away, scrambling after her as she left.
"It's not fair, you can't hide anywhere here, and you know that you're faster than me!"
"Less talking, more chasing!"
Luna could only stare in dumb amazement as her sister got up on her hooves with a groan, and began to run after Nightmare Moon; she couldn't help but stare as her sister tried (And failed) to keep up with Nightmare Moon, who was laughing and taunting at her sister. After about five minutes of watching an obviously exhausted Celestia try to keep up with her, she could only say one thing about the whole ordeal, though rather quietly, so not to disturb the two.
"You have weird nightmares, Tia!"
Shaking her head in disappointment, she decided to leave this nightmare unattended to, and left her sister's dream, determined to help a little filly or colt from nightmares about Slendermane; at least that was a realistic nightmare, not...Whatever her sister was dreaming of right now. That, and her sister could use the exercise, even if it was from playing tag with Nightmare Moon.